This invention generally relates to electro deposition processes of gallium and gallium alloy films for fabrication of thin film photovoltaic devices such as those containing copper, indium, gallium, and/or selenium and as thermal interface materials.
For photovoltaic applications, two layers of semiconductor material having different characteristics are generally used in order to create an electrical field and a resultant electrical current. The first layer is typically an n-type semiconductor material and is generally thin so as to let light pass through to an underlying p-type semiconductor layer material layer, which is often referred to as the absorbing layer. The absorbing layer in combination with the n-type semiconductor material layer provides a suitable band gap to absorb photons from the light source and generate a high current and an improved voltage. For the p-type layer, thin films of a copper-indium-gallium-diselenide semiconductor material (i.e., CuInGaSe2 and variations thereof, also referred to as CIGS) or copper indium diselenide (i.e., CuInSe2, also referred to as CIS) or copper gallium diselenide (i.e., CuGaSe2, also referred to as CGS) have generated significant interest over the years for their use in photovoltaic devices.
By way of example, the p-type CIGS layer is typically combined with an n-type CdS layer to form a p-n heterojunction CdS/CIGS device. Zinc oxide and doped zinc oxide may be added to improve transparency. The direct energy gap of CIGS results in a large optical absorption coefficient, which in turn permits the use of thin layers on the order of 1-2 μm. By way of example, it has been reported that the absorbed layer band gap was increased from 1.02 electron-volts (eV) for a CuInSe2 (CIS) semiconductor material to 1.1-1.2 eV by partial substitution of the indium with gallium, leading to a substantial increase in efficiency.
Formation of the CIGS structure is typically by vacuum deposition, chemical deposition or electrodeposition. The most common vacuum-based process co-evaporates or co-sputters copper, gallium, and indium, then anneals the resulting film with a selenium or sulfur containing vapor to form the final CIGS structure. An alternative is to directly co-evaporate copper, gallium, indium and selenium onto a heated substrate. A non-vacuum-based alternative process deposits nanoparticles of the precursor materials on the substrate and then sinters them in situ. Electrodeposition is another low cost alternative to apply the CIGS layer. Although electrodeposition is an attractive option for formation of gallium thin films, especially for photovoltaic applications such as CIGS, current processes are generally not commercially practical. Gallium is generally considered a difficult metal to deposit without excessive hydrogen generation on the cathode because the gallium equilibrium potential is relatively high. Hydrogen generation on the cathode causes the deposition efficiency to be less than 100% because some of the deposition current gets used to form hydrogen gas rather than to form the gallium film on the substrate or cathode. Low cathodic deposition efficiency due to excessive hydrogen generation results in poor process repeatability, partly due to the poor cathodic efficiency, and most importantly to poor deposit film quality with high surface roughness and poor deposit morphology.
Accordingly, there is a need in the art for improved electrodeposition processes for depositing gallium and gallium alloys as well as novel photovoltaic devices containing the same with increased band gap to provide increased photovoltaic current.